DEC Rainbow

Rainbow 100

DEC Rainbow 100 as a museum piece in Citilab
Type	Personal computer
Release date	1982; 35 years ago (1982)
Operating system	CP/M, MS-DOS, UCSD p-System, Concurrent CP/M, Venix
CPU	Zilog Z80 @ 4.012 MHz and Intel 8088 @ 4.815 MHz
Memory	64 - 896 kB
Successor	VAXmate

The Rainbow 100 was a microcomputer introduced by Digital Equipment Corporation (DEC) in 1982. This desktop unit had a monitor similar to the VT220 in a dual-CPU box with both 4 MHz Zilog Z80 and 4.81 MHz Intel 8088 CPUs. The Rainbow 100 was a triple-use machine: VT100 mode (industry standard dumb terminal for interacting with DEC's own VAX), 8-bit CP/M mode (using the Z80), and 16-bit CP/M-86 or MS-DOS mode using the 8088.

The Rainbow came in three models, the 100A, 100B and 100+. The "A" model was the first released, followed later by the "B" model. The most noticeable differences between the two models were the firmware and slight hardware changes. The systems were referred to with model numbers PC-100A and PC-100B respectively; later "B" models were also designated PC-100B2.

The "A" model was the first produced by Digital. The distinguishing characteristic of the "A" model from an end-user perspective was that the earlier firmware did not support booting from a hard disk. Other distinguishing hardware features included the three 2764 ROM chips holding the system firmware and the case fan/power supply combinations. Versions of the 100A shipped outside the USA included a user-changeable ROM chip in a special casing. The user changed the built-in ROM for this one to support their keyboard layout and language of the boot screen.

The "B" model followed the "A" model, and introduced a number of changes. The "B" model featured the ability to boot from a hard disk (referred to as the Winchester drive) via the boot menu due to updated firmware. The hardware changes included bigger firmware stored on two 27128 ROMs (16 kB) and an improved case fan/power supply. The firmware allowed selection of the boot screen language and keyboard layout, eliminating the need to switch ROM.

The "100+" model was actually a marketing designation signifying that the system shipped with a hard drive installed; the "100+" and "B" models were identical in all other respects. When a hard-disk option was installed on the Rainbow, the kit included the 100+ emblem for the computer's case.